The present invention relates to communications among a plurality of spacecraft using determined receive beams.
Differently configured antennas have been developed to achieve desired objectives. In one area of application, antennas have been devised for use with spacecraft that orbit the earth. The orbiting spacecraft using their antennas are able to gather and/or process information, such as data related to events or conditions on the earth""s surface.
In one specific class associated with antennas that orbit the earth, there are cross-link antenna systems which have been configured for communication and/or navigation purposes. These cross-link antenna systems include the capability of providing communications among a plurality of spacecraft. A particular type of cross-link system includes global positioning system (GPS) satellites. The GPS is used to provide central calibration of all spacecraft in the particular constellation to the same time reference. In addition, this cross-link system sends a given signal from one spacecraft to all other spacecraft in the constellation to allow dissemination of information to one or more of the spacecraft that is in view of a part of the earth that is of current interest. Known cross-link antenna systems in the GPS constellation use hemispherical patterns in both receive and transmit modes during communications among the satellites of the constellation. Each satellite transmits in a prescribed time bin of 1.5 seconds and all other spacecraft receive this information during that time interval. This scheme has a drawback in that the antennas with the satellites of the GPS have relatively small gain. This limits the data range and more importantly makes this cross-link system susceptible to interference due to poor link margin.
With reference to FIG. 1, the prior art cross-link system is diagrammatically illustrated. The prior art system includes a number of spacecraft and, at any instance in time, one of the spacecraft can be characterized as the transmitting spacecraft 20 and the other spacecraft of the constellation can be characterized as the receiving spacecraft 24a, 24b, 24c, 24d. As schematically represented, the transmitting spacecraft 20 sends a multi-directional transmit beam when it is in its transmit mode. Each of the other spacecraft of the constellation, the receiving spacecraft 24a-24d, is in its receive mode and each generates or outputs a receive beam for receiving the transmit beam, during the time interval that the transmitting spacecraft 20 is in its transmit mode. As also represented schematically, each of the receiving spacecraft 24a-24d generates the same shape or configuration of receive beam, namely, the hemispherical pattern. Further depicted in FIG. 1 is the ever present interference or noise emanating from the earth (E). This noise, which is picked up by the hemispherical patterns of the receive beams of the receiving spacecraft, negatively impacts the transmission being sent by the transmitting spacecraft 20. In accordance with this prior art system, once the transmitting spacecraft 20 has finished with its transmit mode, with the prescribed time bin for that spacecraft ending, another spacecraft in the constellation enters its transmit mode. More specifically, one of the previously identified receiving spacecrafts 24a-24d now becomes the transmitting spacecraft. The other spacecraft including the former transmitting spacecraft are placed in their receive modes to receive the transmit beam from this next transmitting spacecraft during communications, such as establishing the same time reference for each of the spacecraft in the constellation. For each time bin with a different transmitting spacecraft, the same non-configurable beam reception patterns are generated by the receiving satellite that have unwanted results including signal interference and data range limitations.
In accordance with the present invention, a cross-link system is provided that includes a plurality of spacecraft. Each of the spacecraft has an antenna. One or more of these antennas can point or direct their receive beams in the direction of the transmit beam emanating from the current transmit antenna of the transmit spacecraft. This control of the receive beams improves the link margin due to the increased gain of such currently receiving antennas and their lower sidelobes in the unwanted direction(s). Control over beam direction can be done either adaptively or on a fixed basis.
With respect to the one or more antenna apparatuses, that are part of spacecraft in the constellation, and which can control its receive beam in the direction of the current transmit beam, each such antenna apparatus can include a beam controller, phase shift circuitry, a plurality of low noise amplifiers (LNAs), a number of antenna or radiating elements, a combiner and a receiving radio. The beam controller includes at least one processor involved with outputting signals that control the amplitudes and phases of the antennas elements. The phase shift circuitry preferably includes a number of phase shifters. Each of the different phase shifters electrically communicates with a different one of the LNAs. Each LNA electrically communicates with a particular one of the antenna elements. The control signals from the beam controller enable different signal amplitudes to be individually applied to the antenna elements after amplification by its associated LNA. Each of the phase shifters can be separately activated at different times. Depending on the direction of the receive beam for the antenna of a particular spacecraft, a desirably directed receive beam can be output by that antenna apparatus, namely, in the direction of the transmit beam from the transmit antenna that is currently generating the transmit beam. A combiner signal is developed in the combiner, which is a combination of the signals received using the antenna elements, LNAs and phase shifters due to the generated receive beam. The combiner signal is output to the receiving radio. In one embodiment, the receiving radio is involved with a determination related to the signal-noise ratio (SNR) of the combiner signal. The SNR of the combiner signal is useful in ascertaining a maximum, preferred or desired gain of the receive beam for a particular transmit beam.
Regarding communications involving a spacecraft of the particular constellation when one of them is in a transmit mode and the other spacecraft are in a receive mode, a description is provided related to a communication between the current transmit spacecraft and one of the spacecraft that is receiving the transmit beam from this transmit spacecraft. As can be appreciated, this description also applies to other spacecraft in the constellation that are generating their own receive beams to receive the current transmit beam. This description is also applicable to subsequent transmit beams from other spacecraft in the constellation when they are caused to send a transmit beam during a specified time bin.
Important to controlling a first receive beam associated with a first antenna of a first spacecraft are making determinations related to locations of each of the first receive spacecraft and the first transmit spacecraft that has the first transmit antenna outputting the first transmit beam. Additionally, location information is obtained related to the antenna elements of the first receive antenna.
With regard to the location of the first receive spacecraft, first and second receive values are obtained. Preferably, the first and second receive values are angle related. In one embodiment, the first receive value relates to a first angle in one plane, such as the elevation plane. The second receive value relates to a second angle in the azimuth plane. First and second attitude values are also obtained related to the attitude of the first receive spacecraft. The first attitude value relates to one attitude angle in elevation and the other attitude value relates to the attitude of the first receive spacecraft in azimuth. As is well known, the attitude of a spacecraft involves three axes, namely, yaw, pitch and roll axes. In one embodiment, only the yaw axis in each of the elevation and azimuth planes is taken into account in obtaining the first and second attitude values. In such an embodiment, the impact or significance of attitude in the pitch and roll axes is sufficiently small to be ignored.
Using such location information based on elevation, azimuth and attitude angles for the first receive spacecraft and elevation and azimuth angles for the first transmit spacecraft, first and second steering angles can be determined, which can be identified as Phi ("PHgr") and Theta ("THgr"). These first and second steering angles ("PHgr" and "THgr")) are utilized with location information of the antenna elements to determine, for each of the antenna elements of the first receive antenna, phase information that can be used in energizing the antenna elements to generate the desired first receive beam. In one embodiment, the antenna element location information that is utilized includes both distance and angle information. In the case of a first receive antenna, a first distance is obtained related to a distance that the first antenna element is located from the center of the array of the antenna elements for the first antenna. In one embodiment, the angle information relates to the angle defined by the first antenna element relative to the center of the antenna element array. In an even more specific embodiment, the number of antenna elements is seven. One of the seven antenna elements is positioned in the center of the antenna element array and the other six antenna elements are disposed radially outwardly from the center antenna element, whereby each of the six antenna elements is angularly equally spaced from its adjacent antenna element by 60xc2x0.
The phase information for each antenna element in the array can be determined using the same first and second steering angles, while also using the different angle information and the same or different distance information associated with each of the antenna elements. With respect to amplitude control, in one embodiment, the amplification factor associated with each of the LNAs is the same. The amplitude control signal from the beam controller can be selected or determined so that the amplitude associated with each antenna element is close to, but does not result in, a saturated antenna element signal. After obtaining the phases and the amplitudes for each of the antenna elements (e.g., seven antenna elements), a first receive beam for the first receive antenna can be generated by energizing the antenna elements according to such phase and amplitude values.
In a preferred embodiment, instead of utilizing only the set made up of these determined phases and amplitudes, variants of them are calculated. Such variant values can be a percentage or predetermined magnitude of the previously determined phase and amplitude values. Substantial numbers of different combinations of variant phase values and variant amplitude values are obtained, such as and by way of example five different variants for each antenna element on each of the positive and negative sides of the determined or selected phases and amplitudes. Each variant is used in a process that involves generating receive beams based on one or more variants. Using each such generated receive beam, the combiner develops a combiner signal. The combiner signal is representative of the vectorial sum of the antenna element signals and can be used by the receiving radio to evaluate the SNR of the combiner signal for the particular receive beam. The SNR of the combiner signal relates to the ratio of the vectorial sum of the fields associated with the antenna elements in the direction of the first transmit spacecraft to the vectorial sum of the fields associated with the antenna elements in a direction of interference. According to this preferred embodiment, it is desired that a receive beam be found that results in the highest or greatest ratio of these two vectorial sums. In one embodiment, this is determined by an analysis of the SNRs for each of the receive beams that are generated using the different variants. Each of the determined SNRs can be evaluated including compared with each other to determine the desired, preferably maximum, SNR for all these different receive beams.
After the phase and amplitude variants have been determined and, along with the previously and initially determined phase values and amplitude values, have been analyzed to determine which thereof provides the maximum, or desired, ratio (e.g., optimum SNR), the phase values and amplitude values that correspond to this ratio can be utilized to generate the first receive beam that is used in gathering information from the transmit beam. These phases and amplitudes constituting the maximum or desired ratio provide the desired or maximum antenna gain in the direction of the transmit spacecraft while providing a null, or substantially close thereto, in a direction of interference for the first antenna.
In another embodiment, in addition to making the determinations and/or selections for the phase values and amplitude values for each of the antenna elements in the direction of the transmit spacecraft, which values are intended to result in the desired or maximum gain for the receive antenna, a second gain determination is made using a fixed location(s) on earth. This second gain determination is different from that made when determining the direction of interference in the previous embodiment where the direction of interference is unknown. The objective is to maximize the ratio of the gain in the direction of the transmit spacecraft to the gain in the direction of the fixed location(s). Essentially, it is desirable that the receive antenna generate a receive beam that provides a null, or substantially a null, in the direction of the fixed location(s). Because each such fixed location is known, a determination can be made of the phases and amplitudes that need to be used in achieving the null or substantial null in each fixed location. In particular, angle information associated with each fixed location, together with the requirement that the combiner output a combiner signal that includes a null in the fixed direction, enables the analysis to be conducted that results in the determined phases and amplitudes that, among other things, provide the null or substantial null in the direction of each fixed location.
In still another embodiment that is related to the embodiment in which it is desired to minimize, or at least reduce, interference in an unknown direction, while achieving a receive antenna gain in the direction of the transmit spacecraft that provides the maximum ratio of the two gains, a further determination is made related to the location of the direction of interference. Since the desired phases and amplitudes were obtained that maximize, or substantially maximize, the ratio of the gains in the direction of the transmit spacecraft and in the direction of interference, such phase and amplitude information can be utilized with the previously defined relationships including the location information associated with the antenna elements, to determine location information for the direction of interference. Since location information is known for the receive spacecraft and the previously determined maximum or desired phase and amplitude values are also known, the location information in the form of angle location information ("THgr", "PHgr") can be found for the previously unknown direction of interference.
Based on the foregoing summary, a number of salient aspects of the cross-link antenna system of the present invention are identified. Receive antenna gain is increased (e.g., greater than 7 dB). A greater rejection of unwanted signals, such as noise or other interference, can be achieved (e.g., greater than 20 dB). This results in an overall gain or improvement of 27 dB or more. The cross-link system is compatible with previous or existing spacecraft that do not include the antenna apparatus of the present invention. That is, spacecraft using the antenna apparatus of the present invention can be in the same constellation as spacecraft that do not have the novel antenna apparatus. Ground interference can be rejected on a pre-programmed or adaptive basis. Hardware changes to previous antenna designs used with cross-link system spacecraft are minimal, namely, insertion of controllable phase shifters and amplifiers in communication with existing antenna elements, together with the use of a combiner, radio receiver and beam controller.